A gas turbine engine commonly requires numerous sensors and servomechanisms to control its operation. Proper functioning of these sensors and servomechanisms requires a highly stable and precise electrical power supply. Typically, the power supply utilized is one designated as a tracking voltage regulator. The regulator receives electric power from a generator or alternator (the term generator is used herein as including both) which is mechanically driven by the engine itself. Such a regulator normally supplies two voltages of opposite polarity and equal magnitude. The regulator is designated as tracking because one of the voltages is constrained to follow, or track, the other in magnitude; however, the latter is generally independent of, and need not track, the former. Such a regulator frequently includes two stages, the first of which receives electrical power from the generator and regulates the voltage with a relatively coarse degree of accuracy. The second stage receives power from the first stage and regulates the voltage with a relatively finer degree of accuracy. The finely regulated voltage is used to energize the engine's sensors and servomechanisms.
In general, the finely regulated voltage stage of such power supplies can supply only a limited amount of current. If this stage should attempt to supply a greater current, as it may attempt to do when a short circuit occurs in a sensor or servomechanism which it energizes, the finely regulated voltage supplied will tend to drop in magnitude. This failure in voltage stability of the tracking supply has serious consequences to the sensors and servomechanisms. For example, during such a voltage drop, a sensor which measures engine speed may give an incorrectly low reading. A speed control servomechanism which relies on this sensor may attempt to accelerate the engine in response with the result that an engine overspeed condition may occur. Alternately, a servomechanism which controls an airflow pattern may attempt to change the pattern with the result that a condition of excessive temperature may occur.
Another problem can arise in the situation wherein the finely regulated voltage itself is used to energize a system which monitors that same voltage. In such a case, a drop in the regulated voltage can affect the circuitry of the monitoring system in such a way that the drop is, in effect, self-cancelling with the result that no drop is reported by the monitor. This can occur, for example, in a case where the monitor generates a reference voltage which is dependent upon the voltage monitored and then compares the two voltages to determine whether the monitored voltage is correct. A situation can occur in which a drop in the monitored voltage produces a corresponding drop in the reference voltage so that no change is detected in the difference between the two. In such a case, the power supply voltage drop would arise and persist undetected.
A further problem can occur in power supply monitors of this type during the startup cycle of the engine. During this cycle, the generator which produces the electricity which powers the power supply produces power at a relatively low voltage. Accordingly, the output voltage provided by the power supply will be relatively low. The power supply monitor can read this low voltage as indicative of a harmful condition when, in fact, it merely results from the operation of the engine in a startup mode.